The present invention relates to a clamping device for machine tools comprising a chuck and a drawbar.
Clamping devices for machine tools are generally known. The European patent application EP 255 042 A1 discloses for instance such a device.
The EP 255 042 A1 describes a clamping device known to the market as the “Macro chuck” of the applicant System 3R. It uses clamping balls to lock the drawbar of the pallet in the clamped position. The drawbar is loosely connected to the pallet by a bayonet connection. The drawbar is further loosely inserted into the chuck and only held by the clamping balls at clamping. This ensures that no side forces are exerted on the pallet at the moment of clamping. Side forces generally have a negative effect on the accuracy of the clamping system, generating positioning errors. It is a prerequisite for the invention to ensure a high position accuracy.
The Macro chuck described in the EP 255 042 A1 has a very strong holding force, generated from the installed springs as from the simultaneously and additionally exerted pneumatic force applied on the clamping balls and consequently on the draw bar (this feature of the Macro chuck is the so called turbo function). In some installations like turning machines without an air swivel etc. it is however not possible to apply an additional pneumatic force during the machining process, i.e. compressed air is not available.
Another publication, the EP 2 052 808 A1, describes a clamping chuck with sensing function for detection of locked or unlocked status. The sensing function detects the position of the clamping elements. The sensing function is in itself also a possible source of failure.
The EP 195 29 22 A1 describes a clamping chuck having a number of wedge elements for locking workpiece holder. By using wedge elements instead of e.g. clamping balls the contact surface is increased. By that, the maximum holding force is said to be increased. This chuck however doesn't make use of a drawbar but directly clamps workpiece holder resp. its specific pallet for workpieces. A loosely connected drawbar is however a prerequisite for achieving a high position accuracy of the pallet. The invention described afterwards maintain the loose connection between the drawbar and the separate pallet to be clamped.
Similar clamping devices like the EP 195 29 22 A1 functioning without drawbar are disclosed in the documents U.S. Pat. No. 5,190,272 or EP 1 068 919 A1.
The devices of EP 195 2922 A1 and the other foregoing mentioned publications might have their advantages, but—apart the mentioned positioning accuracy and space problem—can cause potentially severe safety problems, since the wedge elements are just spring loaded in the clamped position and—in case the spring load should result in being insufficient—the pallet is not mechanically fully safe locked into the chuck.
A further disadvantage of all forgoing mentioned clamping devices relies in their storage and handling ability: The Macro system disclosed in the EP 255 042 A1 has the drawbar preconnected to the pallet. This is of advantage for example for the storage of hanging EDM electrodes. For that, grooves in the drawbar are used for storage in fork receivers in the magazine. Other grooves in the drawbar are used for the gripping by a pallet changer. However, for standing pallets a preconnected drawbar is a disadvantage. Standing pallets are often loaded in table chucks in milling machines or in horizontal spindle chucks in turning machines. A preconnected drawbar is an obstacle for an easy handling of the loading process in both cases. Further on, the distance between the magazine shelves in the pallet changer has to be longer when the drawbar is preconnected to the pallet (see e.g. EP 195 2922 A1, functioning without drawbar but with a specific pallet of long dimension). This is in order not to collide with the robot gripper. The result is a shorter allowable height of the workpieces, leading to a limitation of the capacity of the pallet changer.
Pallets like the ones published in the documents U.S. Pat. No. 5,190,272 or EP 1 068 919 A1 allow a shorter distance between the shelves. However, this type of pallet systems do not meet the requirements of clamping without side forces. This since they don't have a loosely connected drawbar or similar solution to transmit the clamping force from the chuck to the pallet in a pure axial way.
Clamping chucks must hold the workpiece pallet with a sufficiently high pull-in force with respect to the external forces e.g. cutting forces from machining. This is not a problem at machining of EDM electrodes as the copper or graphite are of soft materials. In other applications such as turning and milling machines for the working of steel parts sudden high forces can occur, especially at a tool breakage. In such cases the workpiece and also the pallet could be damaged. Despite this, for safety reasons it has to be ensured that in the worst case the pallet holding the workpiece can never get lost from the chuck. This is a main issue for the present application.
It is an object of the present invention to provide a clamping system with an improved storage and handling ability and additionally a safety locking function ensuring that the pallet cannot get lost from the chuck in case the ordinary operational clamping force is overcome for whatever reason (e.g. due to an incident like a tool breakage or other circumstances that will lead to very high external forces overcoming the ordinary clamping mechanism). This objective shall also be reached in devices using loosely connected drawbars to transmit the clamping force from the chuck to the pallet in a pure axial way or situations were compressed air is not available.